The development of methodology to incorporate high silicone loadings in the soft segment of polyurethanes has resulted in the production of biostable polyurethanes (Elast-Eon™). The soft segment of these polyurethanes is based on 80 wt % of a hydroxyl terminated polydimethylsiloxane (PDMS) and 20 wt % of a polyether polyol specifically polyhexamethylene oxide (PHMO).
It was thought that the presence of the polyether was necessary in order to act as a compatibiliser between thermodynamically diverse molecules of the isocyanate rich hard segment and the silicone rich soft segment. Polyurethanes with silicone contents higher than 80 wt % of the soft segment resulted in poorer mechanical properties which at that time seemed to corroborate the compatibiliser theory.
The technology of Elast-Eon production has more recently evolved leading to significant process breakthroughs that allow the incorporation of higher silicone content into the soft segment without a corresponding decrease in mechanical properties.
A considerable amount of work has also been done in understanding the biologically induced degradation mechanisms in polyurethanes. Oxidising radicals emanating from foreign body giant cells (FBGC) in the vicinity of the implant are seen to be the major source of degradation of polyurethane based medical devices. As a result, it has been shown that the presence of oxidisable groups in polyurethanes are the primary sites of the initiation of degradation. Therefore, a reduction in the content of groups prone to oxidation will lead to an increase in the biological stability of the polyurethanes. As a consequence, it is now desirable to have a soft segment which is based on PDMS instead of PDMS in combination with a polyether polyol.